Two or three pages into this
http://www.daddario.com/upload/tension_chart_13934.pdf is the formula for pitch vs strength length and weight. You can get the unit weight for a given string size by looking further into the document. The parameters for the D’Addario strings are pretty close to thosee for other manufacturers, so this information should get you close enough. When I use the formula to help in string size selection, I put it into a spreadsheet so that the arithmetic is automatic and I can make a table of my conjectures. Happy calculations!